The present invention relates to a photothermographic material. In particular, the present invention relates to a photothermographic material for scanners, image setters and so forth, which is particularly suitable for photographic art. More precisely, the present invention relates to a photothermographic material suitable for infrared semiconductor lasers.
There are known many photosensitive materials having a photosensitive layer on a support, with which image formation is attained by imagewise light exposure. These materials include those utilizing a technique of forming images by heat development as systems that can contribute to the environmental protection and simplify image-forming means.
In recent years, reduction of amount of waste processing solutions is strongly desired in the field of photographic art from the standpoints of environmental protection and space savings. Therefore, development of techniques relating to photothermographic materials for photographic art is required, which materials enable efficient exposure by a laser scanner or laser image setter and formation of clear black images having high resolution and sharpness. Such photothermographic materials can provide users with simpler and non-polluting heat development processing systems that eliminate the use of solution-type processing chemicals.
Methods for forming images by heat development are described in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. Klosterboer, xe2x80x9cThermally Processed Silver Systems Axe2x80x9d, Imaging Processes and Materials, Neblette, 8th ed., compiled by J. Sturge, V. Walworth and A. Shepp, Chapter 9, p.279, (1989). Such photothermographic materials comprise a reducible non-photosensitive silver source (e.g., silver salt of an organic acid), a photocatalyst (e.g., silver halide) in a catalytically active amount and a reducing agent for silver, which are usually dispersed in an organic binder matrix. While the photosensitive materials are stable at an ordinary temperature, when they are heated to a high temperature (e.g., 80xc2x0 C. or higher) after light exposure, silver is produced through an oxidation-reduction reaction between the reducible silver source (which functions as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image generated upon exposure. The silver produced from the reaction of the reducible silver salt in the exposed areas shows black color and provides contrast with respect to the non-exposed areas, and thus images are formed.
In many of conventionally known photothermographic materials, image-forming layers are formed by coating a coating solution using an organic solvent such as toluene, methyl ethyl ketone (MEN) and methanol as a solvent. However, not only use of an organic solvent as a solvent adversely affect human bodies during the production process, but also it is disadvantageous in view of cost because it requires process steps for recovery of the solvent and so forth.
Accordingly, methods of forming an image-forming layer by coating a coating solution using water as a solvent have been proposed. For example, Japanese Patent Laid-open Publication (Kokai, hereinafter referred to as JP-A) 49-52626, JP-A-53-116144 and so forth disclose image-forming layers utilizing gelatin as a binder, and JP-A-50-151138 discloses an image-forming layer utilizing polyvinyl alcohol as a binder. Furthermore, JP-A-60-61747 discloses an image-forming layer utilizing gelatin and polyvinyl alcohol in combination. As another example, JP-A-58-28737 discloses an image-forming layer utilizing a water-soluble polyvinyl acetal as a binder. If these binders are used, image-forming layers can be formed by using a coating solution comprising an aqueous solvent, and therefore considerable merits can be obtained with respect to environment and cost.
However, when a polymer such as gelatin, polyvinyl alcohol or water-soluble polyacetal is used as a binder, silver tone of developed areas becomes brown or yellow, which quite differs from black color regarded as a preferred proper color, and in addition, there arise, for example, problems that the blacking density in exposed areas becomes low and the density in unexposed areas becomes high. Thus, there can be obtained only images of which commercial value is seriously impaired. Furthermore, since such polymers show bad compatibility with the silver salt of an organic acid, there may also arise a problem that practically acceptable coatings cannot be obtained in view of coated surface quality.
European Patent Publication (hereinafter referred to as EP-A) 762196, JP-A-9-90550 and so forth disclose that high-contrast photographic property can be obtained by incorporating Group VII or VIII metal ions or metal complex ions thereof into photosensitive silver halide grains for use in photothermographic materials, or incorporating a hydrazine derivative into the photosensitive materials.
Meanwhile, photosensitive materials for platemaking are used as intermediate materials in printing process, i.e., as masks for producing printing plates. In recent years, digitization and automation of operations have been widely diffused for the whole printing process, and light exposure and development of PS plates in the platemaking process are automated by the use of platemaking machines. Such platemaking machines serve as systems that operate when information required for automatic transportation, automatic light exposure and so forth (bar code or register mark) written on photothermographic materials is read by sensors of the platemaking machines. These sensors utilize laser diodes of a wavelength around 670 nm. That is, it is indispensable that the information written on the photothermographic materials can be read at a wavelength of 670 nm, and it is preferred that the photothermographic materials should show low Dmin (minimum density) at a wavelength of around 670 nm. In particular, it is preferable to use an antihalation dye showing low absorption in the visible region.
As for photothermographic materials that are designed to be subjected to infrared light exposure, absorption in the visible region of sensitizing dye or antihalation dye can be significantly reduced, and thus substantially colorless photothermographic materials can easily be produced. Further, semiconductor lasers have been widely used in recent years, and silver halide photosensitive materials for infrared sensitization have become to be used more frequently.
Since laser lights including those of semiconductor lasers show specific emission wavelengths, it is sufficient to strongly sensitize only the characteristic wavelength corresponding to the oscillation wavelengths of lasers. In other wards, in not a few cases, it is rather preferred that sensitivity for the wavelength regions other than the laser oscillation wavelength should be as low as possible in view of safe light safety and so forth. A technique for sensitizing only for a specific wavelength for that purpose is known as J-band sensitization among the spectral sensitization techniques for silver halide photosensitive emulsions. J-band is obtained by formation of particular aggregates called J-aggregates, and shows extremely sharp absorption with extremely strong absorbance and extremely narrow half-width. Reflecting this absorption characteristic, spectral sensitivity also shows a sharp spectral sensitivity distribution spectrum. Many examples of this J-band sensitization are known for visible region and it is an indispensable spectral sensitization technique for the production of full color photosensitive materials, for example, However, examples thereof for the infrared regions are extremely scarce, and only brief explanations are found in A. H. Henry, Photogr. Sci. Eng., vol. 18 (No. 3), pp.323-335, (1974) and H. Rampfer, ICPs Reports, pp.366-369 (1986). Further, examples thereof for photothermographic materials are also scarce.
In infrared sensitization systems showing a sensitization peak at a wavelength longer than 730 nm, on the other hand, an increased addition amount of sensitizing dyes provides strong desensitization (described in U.S. Pat. No. 4,011,083 etc.). This desensitization is well known as proper desensitization provided by sensitizing dyes, and it is also known to become stronger with a dye showing absorption at a longer wavelength. Because such sensitizing dyes for infrared region as mentioned above show extremely strong desensitization of such a kind, the coating ratio of the dyes on the silver halide grain surfaces must be generally around 10-20%. Therefore, the light capturing ratio inevitably becomes low, and spectral sensitivity to be obtained is much lower than the spectral sensitivity imparted for the visible region. Thus, spectral sensitivity distribution obtained in such a state becomes very broad one based on the absorption of the dyes in molecular state. Furthermore, although the aforementioned literature mentions formation of J-band, that spectral sensitization is assumed to consist of both of M-band type spectral sensitization based on molecules of sensitizing dye in the molecular state and J-band type spectral sensitization based on J-aggregates of sensitizing dye, because it is described that an extremely broad spectral sensitivity distribution was obtained with silver chlorobromide emulsion mainly consisting of silver iodobromide and silver bromide. Further, because laser light exposure, handling under a safe light, adaptation to full color photosensitive material and so forth were not well taken into consideration, it was not intended to suppress sensitivity for the unnecessary regions. Therefore, although J-band sensitization was obtained in any case, it was not realized as one mainly consisting of J-band type spectral sensitization with narrow spectral sensitization distribution, and thus it was extremely unsatisfactory one for practical use.
Therefore, there is desired a sensitization method that provides J-band sensitization with high spectral sensitivity suitable for an infrared region, in particular, wavelengths of semiconductor laser lights, and simultaneously provides low sensitivity for unnecessary regions. Further, if spectral sensitization is desired for a wavelength longer than 730 nm, a spectral sensitization dye that can efficiently absorb long wavelength lights must be used. Infrared sensitizing dyes are disclosed in JP-A-61-137149, JP-A-63-197947 and JP-A-55-13505 as well as JP-A-59-191032, JP-A-59-192242 and JP-A-60-80841 mentioned above and so forth, and there are used dyes having long conjugated methine chains in order to provide absorption in an infrared region. However, not only it is extremely difficult to form J-aggregates on silver halide grains with cyanine dyes having such long methane chains to predominantly provide J-band sensitization, but also dyes providing spectral sensitization by infrared absorption generally show high HONG and hence strong reducing ability, and thus they are likely to reduce silver ions in photothermographic materials to degrade fog of the photothermographic materials. In particular, during storage under high temperature and high humidity or storage for a long period of time, marked change of performance may be observed. Moreover, if a dye showing low HOMO is used in order to prevent the degradation of storability, LUMO also correspondingly becomes lower. Thus, spectral sensitization efficiency is reduced and hence sensitivity is lowered. These problems concerning sensitivity, storability and performance fluctuation are observed not only in wet type photosensitive materials, but also in photothermographic materials, in which the problems become more serious.
Therefore, there have been desired techniques for providing photothermographic materials that can provide images with low fog as well as little increase of fog and little sensitivity fluctuation during storage before light exposure, and are advantageous for environment and cost.
Therefore, in view of the aforementioned problems of the prior art, an object of the present invention is to provide a photothermographic material, in particular, for photographic art, especially for scanners and image setters, that can form images with low fog and show little increase of fog and little sensitivity fluctuation during storage before light exposure.
The inventors of the present invention assiduously studied in order to achieve the aforementioned object. As a result, they found that a superior photothermographic material could be obtained if spectral sensitization is performed so that spectral sensitivity or optical density in the range of maximum spectral sensitivity wavelength xc2x130 nm could satisfy particular conditions, and thus accomplished the present invention.
That is, the present invention provides a photothermographic material containing a non-photosensitive silver salt and a photosensitive silver halide on a support, wherein the photosensitive silver halide is spectrally sensitized with a spectral sensitizing dye so that maximum spectral sensitivity wavelength could become longer than 730 nm and the conditions defined by the following formulas (1) and (2) could be satisfied.
300xe2x89xa7S(xcex max)/S(xcex max+30 nm)xe2x89xa74.5xe2x80x83xe2x80x83Formula (1) 
30xe2x89xa7S(xcex max)/S(xcex maxxe2x88x9230 nm)xe2x89xa72xe2x80x83xe2x80x83Formula (2) 
In the above formulas, xcexmax denotes maximum spectral sensitivity wavelength, and S(xcex) denotes spectral sensitivity at a wavelength of xcex.
The present invention also provides a photothermographic material containing a non-photosensitive silver salt and a photosensitive silver halide on a support, wherein the photosensitive silver halide is spectrally sensitized with a spectral sensitizing dye so that maximum spectral sensitivity wavelength could become longer than 730 nm and the conditions defined by the following formulas (3) and (4) could be satisfied.
300xe2x89xa7Abs.(xcex max)/Abs.(xcex max+30 nm)xe2x89xa74.5xe2x80x83xe2x80x83Formula (3) 
30xe2x89xa7Abs.(xcex max)/Abs.(xcex maxxe2x88x9230 nm)xe2x89xa72xe2x80x83xe2x80x83Formula (4) 
In the above formulas, xcexmax denotes is maximum spectral sensitivity wavelength, and Abs. (xcex) denotes optical density at a wavelength of xcex.
The photothermographic material of the present invention shows photographic properties suitable for photographic art, i.e., high sensitivity and little increase of fog and sensitivity fluctuation during long-term storage. Further, it can be prepared by coating an aqueous system, which is advantageous for environment and cost.